Recently, with development of microfabrication in Micro Electro Mechanical Systems technology, various sensors for measurements of physical quantities including accelerations and angular velocities to which materials including silicon and glass are applied have been provided.
The physical sensors may be applied to various structures because three-dimensional structures and movable structures of silicon can be formed according to an advantage that enables formation of a structure having a higher aspect ratio (aperture width/processing depth) than that of a semiconductor device, and a processing method that enables processing of a groove having a higher aspect ratio using dry etching to which an RIE (Reactive Ion Etching) apparatus of an ICP (Induction Coupled Plasma) system is applied, and processing accuracy is higher than that of mechanical processing.
In a structure of a general physical sensor, movable mechanism parts such as oscillators or movable bodies are provided on a silicon substrate and a glass substrate using the Micro Electro Mechanical Systems technology, drive gaps are provided in locations corresponding to the movable mechanism parts including oscillators or movable bodies on a cap substrate, and these substrates are sealed by joining, bonding, or the like. The sizes of these movable mechanism parts are of the order of microns, and a problem of characteristics degradation is caused by influences of air resistance or the like. It is necessary to seal sensing parts in a pressure atmosphere corresponding to the movable mechanism parts including the respective oscillators or movable bodies.
Of the physical sensors, a combo sensor in which an accelerometer and a gyroscope are provided on the same substrate is sealed in a pressure atmosphere in which characteristics of the respective accelerometer and gyroscope are not degraded. Generally, a combo sensor in which characteristics are not degraded by sealing of a sensing part of the accelerometer at atmospheric pressure and sealing of a sensing part of the gyroscope in vacuum is provided.
The gyroscope has an oscillator as the movable mechanism part and when an angular velocity is applied while the oscillator is driven (oscillated) at a natural frequency, a Coriolis force is generated. The oscillator is displaced by the Coriolis force. The angular velocity can be detected by detection of the amount of displacement of the oscillator by the Coriolis force. As the drive speed of the oscillator is higher, the Coriolis force is larger, and it is necessary to oscillate the oscillator at a higher frequency with a larger amplitude for better detection sensitivity of the gyroscope.
However, an oscillator fabricated by the in Micro Electro Mechanical Systems technology is formed with a small gap, and thus, when the oscillation atmosphere is at the atmospheric pressure, the influence of the damping effect of the air (sealed gas) is larger. The damping effect adversely affects the oscillation of the gyroscope at the higher frequency with the larger amplitude, and degrades the detection sensitivity of the gyroscope. Therefore, the sensing part of the gyroscope is sealed in an atmosphere with the less influence of the damping effect, i.e., in vacuum, and thereby, the gyroscope at the higher frequency with the larger amplitude may be obtained. Further, with higher vacuum, the influence of the damping effect is even smaller.
It is important to obtain a stable vacuum atmosphere in the space containing the drive part and the detection part of the gyroscope, and the detection sensitivity of the gyroscope may be stabilized.
On the other hand, the accelerometer has a movable body such as a weight or beam as the movable mechanism part, and the movable body is displaced when an acceleration is applied. The acceleration is detected by detection of the amount of displacement of the movable body. When the accelerometer is sealed in the same vacuum atmosphere as that for the gyroscope, the influence of the damping effect on the movable body of the accelerometer is smaller, and a phenomenon of continued oscillation occurs and disables detection of the acceleration with higher sensitivity. Therefore, the accelerometer is sealed under the atmospheric pressure atmosphere providing the larger damping effect.
There is Patent Literature 1 as a known example of a combo sensor device in which an accelerometer and a gyroscope are combined. In PTL 1, the accelerometer and the gyroscope are integrally formed, the gyroscope is sealed in vacuum, then, an air pressure state for suppression of high-frequency oscillation is formed or a damping agent is enclosed using a ventilation passage formed in a cap substrate at the accelerometer side for changing atmospheres of respective sensor parts, and then, the ventilation passage is sealed by welder or the like. Further, a method of forming a desired vacuum state using a gas venting passage provided in the gyroscope part, and subsequently, blocking the passage to hold the vacuum is described. Anodic bonding is applied.
Or, there is Patent Literature 2 as a known example for holding a vacuum atmosphere of a package electronic part. In PTL 2, with a space formed by an element substrate on which an infrared detection element is provided and a side wall as a first space and a space of a getter (absorbent) substrate provided on the rear side of the element substrate as a second space, both spaces are connected by a hole for increasing the first space.